Pine & Rose Sawflies
A common pest found on evergreen trees this time of year is the sawfly and they can seriously damage conifer trees through defoliation. Several species of sawflies are present in Nebraska, including the European pine sawfly, yellowheaded spruce sawfly and larch sawfly. Most commonly damage is seen on Scotch, Austrian, Ponderosa, Jack and Mugo pine, along with spruce and larch. White pine is rarely damaged.
These insects overwinter as pupae in the soil beneath host trees. In spring the adult insects emerge, resembling small non-stinging, wasp-like insects or flying ants. Females cut slits in evergreen needles with a saw-like apparatus and insert eggs. A new generation of immature insects begins to hatch out from late April through May, and will feed on evergreen foliage until mid-June. The larvae resemble a caterpillar and are light to olive green with a reddish-brown to black head. Larvae usually have several darker green stripes longitudinally down their bodies. There is only one generation of this insect per year.
After hatching out in spring, the larvae feed in large groups on the branches of affected trees. At first the larvae eat only the outer portion of the pine needles, leaving behind the stringy, tough, central needle vein, which dies and turns brown. As the insects mature, they begin consuming entire needles. Due to the coloration of the insects, which blends in well with the color of the needles, the insects are often not noticed until considerable defoliation has already occurred. When disturbed, larvae rear back into a defensive "S" position.
If you're working with a small tree, or only a few larvae are present, then simply knock them off the tree either by hand or with a garden hose. They seldom find their way back up onto the tree and are very susceptible to predators such as birds, rodents and predatory insects.
Large populations should be treated as soon as possible. Several chemical controls can be used, such as insecticidal soaps, horticultural oils, permethrin, bifenthrin, malathion or orthene, but all controls are most effective when the larvae are small. Dipel, or Bacillus thurengiensis, is NOT effective against sawflies because they are in the Hymenoptera family (ants & wasps), not the Lepidoptera family (butterflies & moths).
Another insect that is causing damage right now is the Rose Sawfly. Very similar in life cycle to the Pine Sawfly, this insect skeletonizes and chews holes in the leaves of roses. The larvae hatch out in early May and begin to feed on rose leaves by eating away the plant tissue between the leaf veins. As they grow larger, the insects chew larger holes in the leaves and can cause significant defoliation on your rose plants. Rose sawfly larvae are often found in groups, are medium to light green in color with an orange head and are about 1/2 inch long. There is only one generation of this insect per year.
Control measures include knocking the insects off the plants with a spray of water or picking them off by hand. If many plants are affected, Sevin can be used for control. However, avoid spraying the rose flowers with Sevin because it is highly toxic to bees.